Superhard constructions and methods of making same

ABSTRACT

A superhard polycrystalline construction comprises a body of polycrystalline superhard material having a first region and a second region adjacent to and bonded to the first region by intergrowth of grains of superhard material. The first region comprises a plurality of alternating strata or layers, each having a thickness in the range of around 5 to 300 microns. One or more strata or layers in the second region have a thickness greater than the thicknesses of the individual strata or layers in the first region. The alternating layers or strata in the first region comprise first layers or strata alternating with second layers or strata, the first layers or strata being in a state of residual compressive stress and the second layers or strata being in a state of residual tensile stress. One or more of the layers or strata in the first or second regions comprise a mass of superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, the superhard grains and a non-superhard phase at least partially filling a plurality of the interstitial regions. The median of the mean free path associated with the non-superhard phase divided by (Q 3− Q 1 ) for the non-superhard phase is greater than or equal to 0.50, where Q1 is the first quartile and Q3 is the third quartile; and the median of the mean free path associated with the superhard grains divided by (Q3−Q1) for the superhard grains is less than 0.60.

FIELD

This disclosure relates to superhard constructions and methods of making such constructions, particularly but not exclusively to constructions comprising polycrystalline diamond (PCD) structures attached to a substrate, and tools comprising the same, particularly but not exclusively for use in rock degradation or drilling, or for boring into the earth.

BACKGROUND

Polycrystalline superhard materials, such as polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) may be used in a wide variety of tools for cutting, machining, drilling or degrading hard or abrasive materials such as rock, metal, ceramics, composites and wood-containing materials. In particular, tool inserts in the form of cutting elements comprising PCD material are widely used in drill bits for boring into the earth to extract oil or gas. The working life of superhard tool inserts may be limited by fracture of the superhard material, including by spalling and chipping, or by wear of the tool insert.

Cutting elements such as those for use in rock drill bits or other cutting tools typically have a body in the form of a substrate which has an interface end/surface and an ultra hard material which forms a cutting layer bonded to the interface surface of the substrate by, for example, a sintering process. The substrate is generally formed of a tungsten carbide-cobalt alloy, sometimes referred to as cemented tungsten carbide and the ultra hard material layer is typically polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN) or a thermally stable product TSP material such as thermally stable polycrystalline diamond.

Polycrystalline diamond (PCD) is an example of a superhard material (also called a superabrasive material) comprising a mass of substantially inter-grown diamond grains, forming a skeletal mass defining interstices between the diamond grains. PCD material typically comprises at least about 80 volume % of diamond and is conventionally made by subjecting an aggregated mass of diamond grains to an ultra-high pressure of greater than about 5 GPa, and temperature of at least about 1,200° C., for example. A material wholly or partly filling the interstices may be referred to as filler or binder material.

PCD is typically formed in the presence of a sintering aid such as cobalt, which promotes the inter-growth of diamond grains. Suitable sintering aids for PCD are also commonly referred to as a solvent-catalyst material for diamond, owing to their function of dissolving, to some extent, the diamond and catalysing its re-precipitation. A solvent-catalyst for diamond is understood be a material that is capable of promoting the growth of diamond or the direct diamond-to-diamond inter-growth between diamond grains at a pressure and temperature condition at which diamond is thermodynamically stable. Consequently the interstices within the sintered PCD product may be wholly or partially filled with residual solvent-catalyst material. Most typically, PCD is often formed on a cobalt-cemented tungsten carbide substrate, which provides a source of cobalt solvent-catalyst for the PCD. Materials that do not promote substantial coherent intergrowth between the diamond grains may themselves form strong bonds with diamond grains, but are not suitable solvent-catalysts for PCD sintering.

Cemented tungsten carbide which may be used to form a suitable substrate is formed from carbide particles being dispersed in a cobalt matrix by mixing tungsten carbide particles/grains and cobalt together then heating to solidify. To form the cutting element with an ultra hard material layer such as PCD or PCBN, diamond particles or grains or CBN grains are placed adjacent the cemented tungsten carbide body in a refractory metal enclosure such as a niobium enclosure and are subjected to high pressure and high temperature so that inter-grain bonding between the diamond grains or CBN grains occurs, forming a polycrystalline ultra hard diamond or polycrystalline CBN layer.

In some instances, the substrate may be fully cured prior to attachment to the ultra hard material layer whereas in other cases, the substrate may be green, that is, not fully cured. In the latter case, the substrate may fully cure during the HTHP sintering process. The substrate may be in powder form and may solidify during the sintering process used to sinter the ultra hard material layer.

Ever increasing drives for improved productivity in the earth boring field place ever increasing demands on the materials used for cutting rock. Specifically, PCD materials with improved abrasion and impact resistance are required to achieve faster cut rates and longer tool life.

Cutting elements or tool inserts comprising PCD material are widely used in drill bits for boring into the earth in the oil and gas drilling industry. Rock drilling and other operations require high abrasion resistance and impact resistance. One of the factors limiting the success of the polycrystalline diamond (PCD) abrasive cutters is the generation of heat due to friction between the PCD and the work material. This heat causes the thermal degradation of the diamond layer. The thermal degradation increases the wear rate of the cutter through increased cracking and spalling of the PCD layer as well as back conversion of the diamond to graphite causing increased abrasive wear.

Methods used to improve the abrasion resistance of a PCD composite often result in a decrease in impact resistance of the composite. There is a need for a PCS composite that has good or improved abrasion resistance, fracture and impact resistance and a method of forming such composites.

SUMMARY

Viewed from a first aspect there is provided a superhard polycrystalline construction comprising a body of polycrystalline superhard material, the body of polycrystalline superhard material comprising a first region and a second region adjacent the first region, the second region being bonded to the first region by intergrowth of grains of superhard material; the first region comprising a plurality of alternating strata or layers, each stratum or layer having a thickness in the range of around 5 to 300 microns; the second region comprising a plurality of strata or layers, one or more strata or layers in the second region having a thickness greater than the thicknesses of the individual strata or layers in the first region, wherein:

the alternating layers or strata in the first region comprise first layers or strata alternating with second layers or strata, the first layers or strata being in a state of residual compressive stress and the second layers or strata being in a state of residual tensile stress;

one or more of the layers or strata in the first or second regions comprises:

-   -   a mass of superhard grains exhibiting inter-granular bonding and         defining a plurality of interstitial regions therebetween, the         superhard grains having an associated mean free path; and     -   a non-superhard phase at least partially filling a plurality of         the interstitial regions and having an associated mean free         path;

the median of the mean free path associated with the non-superhard phase divided by (Q3−Q1) for the non-superhard phase being greater than or equal to 0.50, where Q1 is the first quartile and Q3 is the third quartile; and

-   -   the median of the mean free path associated with the superhard         grains divided by (Q3−Q1) for the superhard grains being less         than 0.60.

Viewed from a second aspect there is provided a superhard polycrystalline construction comprising a first region and a second region adjacent the first region, the second region being bonded to the first region by intergrowth of diamond grains; the first region comprising a plurality of alternating strata or layers, each layer or stratum in the first region having a thickness in the range of around 5 to 300 microns; one or more of the layers or strata in the first region and/or the second region comprises:

-   -   a mass of superhard grains exhibiting inter-granular bonding and         defining a plurality of interstitial regions therebetween, the         superhard grains having an associated mean free path; and     -   a non-superhard phase at least partially filling a plurality of         the interstitial regions and having an associated mean free         path;     -   the median of the mean free path associated with the         non-superhard phase divided by (Q3−Q1) for the non-superhard         phase being greater than or equal to 0.50, where Q1 is the first         quartile and Q3 is the third quartile; and     -   the median of the mean free path associated with the superhard         grains divided by (Q3−Q1) for the superhard grains being less         than 0.60.

Viewed from a third aspect, there is provided a method of forming a superhard polycrystalline construction, comprising:

-   -   providing a mass of grains of superhard material and arranging         the mass of superhard grains to form a first region comprising a         plurality of alternating strata or layers, each stratum or layer         having a respective first fraction having a first average size         and a second fraction having a second average size, and         providing a further mass of grains of superhard material to form         a second region adjacent the first region to form a pre-sinter         assembly; and     -   treating the pre-sinter assembly in the presence of a         catalyst/solvent material for the superhard grains at an         ultra-high pressure of around 5.5 GPa or greater and a         temperature at which the superhard material is more         thermodynamically stable than graphite to sinter together the         grains of superhard material to form a polycrystalline superhard         construction, the superhard grains exhibiting inter-granular         bonding and defining a plurality of interstitial regions         therebetween, a non-superhard phase at least partially filling a         plurality of the interstitial regions;     -   the second region being bonded to the first region by         intergrowth of grains of superhard material; the first region a         thickness in the range of around 5 to 300 microns; wherein:     -   the alternating layers or strata in the first region comprise         first layers or strata alternating with second layers or strata,         the first layers or strata being in a state of residual         compressive stress and the second layers or strata being in a         state of residual tensile stress;     -   the median of the mean free path associated with the         non-superhard phase divided by (Q3−Q1) for the non-superhard         phase is greater than or equal to 0.50, where Q1 is the first         quartile and Q3 is the third quartile of the mean free path         measurements associated with the non-superhard phase; and     -   the median of the mean free path associated with the superhard         grains divided by (Q3−Q1) for the superhard grains is less than         0.60, where Q1 is the first quartile and Q3 is the third         quartile of the mean free path measurements associated with the         superhard grains.

Viewed from a further aspect there is provided a tool comprising the superhard polycrystalline construction defined above, the tool being for cutting, milling, grinding, drilling, earth boring, rock drilling or other abrasive applications.

The tool may comprise, for example, a drill bit for earth boring or rock drilling, a rotary fixed-cutter bit for use in the oil and gas drilling industry, or a rolling cone drill bit, a hole opening tool, an expandable tool, a reamer or other earth boring tools.

Viewed from another aspect there is provided a drill bit or a cutter or a component therefor comprising the superhard polycrystalline construction defined above.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described by way of example and with reference to the accompanying drawings in which:

FIG. 1 is a schematic perspective view of an example PCD cutter element for a drill bit for boring into the earth;

FIG. 2 shows a schematic cross-section view of an example of a portion of a PCD structure;

FIG. 3 shows a schematic longitudinal cross-section view of an example of a PCD element;

FIG. 4 shows a schematic longitudinal cross-section view of an example of a PCD element;

FIG. 5 shows a schematic perspective view of part of an example of a drill bit for boring into the earth;

FIG. 6A shows a schematic longitudinal cross-section view of an example of a pre-sinter assembly for a PCD element;

FIG. 6B shows a schematic longitudinal cross-section view of an example of a PCD element;

FIGS. 7A, 7B, 7C and 7D show schematic cross-section views of parts of examples of PCD structures;

The same references refer to the same general features in all the drawings.

DESCRIPTION As used herein, a “superhard material” is a material having a Vickers hardness of at least about 28 GPa. Diamond and cubic boron nitride (cBN) material are examples of superhard materials.

As used herein, a “superhard construction” means a construction comprising a body of polycrystalline superhard material. In such a construction, a substrate may be attached thereto or alternatively the body of polycrystalline material may be free-standing and unbacked.

As used herein, polycrystalline diamond (PCD) is a type of polycrystalline superhard (PCS) material comprising a mass of diamond grains, a substantial portion of which are directly inter-bonded with each other and in which the content of diamond is at least about 80 volume percent of the material. In one embodiment of PCD material, interstices between the diamond grains may be at least partly filled with a binder material comprising a catalyst for diamond. As used herein, “interstices” or “interstitial regions” are regions between the diamond grains of PCD material. In embodiments of PCD material, interstices or interstitial regions may be substantially or partially filled with a material other than diamond, or they may be substantially empty. PCD material may comprise at least a region from which catalyst material has been removed from the interstices, leaving interstitial voids between the diamond grains.

As used herein, PCBN (polycrystalline cubic boron nitride) material refers to a type of superhard material comprising grains of cubic boron nitride (cBN) dispersed within a matrix comprising metal or ceramic. PCBN is an example of a superhard material.

A “catalyst material” for a superhard material is capable of promoting the growth or sintering of the superhard material.

The term “substrate” as used herein means any substrate over which the ultra hard material layer is formed. For example, a “substrate” as used herein may be a transition layer formed over another substrate. Additionally, as used herein, the terms “radial” and “circumferential” and like terms are not meant to limit the feature being described to a perfect circle.

As used herein, the term “integrally formed” regions or parts are produced contiguous with each other and are not separated by a different kind of material.

The superhard construction 1 shown in the FIG. 1 may be suitable, for example, for use as a cutter insert for a drill bit for boring into the earth.

Like reference numbers are used to identify like features in all drawings.

In an embodiment as shown in FIG. 1, a cutting element 1 includes a substrate 10 with a layer of ultra-hard material 12 formed on the substrate 10. The substrate may be formed of a hard material such as cemented tungsten carbide. The ultra-hard material may be, for example, polycrystalline diamond (PCD), polycrystalline cubic boron nitride (PCBN), or a thermally stable product such as thermally stable PCD (TSP). The cutting element 1 may be mounted into a bit body such as a drag bit body (not shown). The exposed top surface of the ultra-hard material opposite the substrate forms the cutting face 14, which is the surface which, along with its edge 16, performs the cutting in use.

At one end of the substrate 10 is an interface surface 18 that interfaces with the ultra-hard material layer 12 which is attached thereto at this interface surface. As shown in the embodiment of FIG. 1, the substrate 10 is generally cylindrical and has a peripheral surface 20 and a peripheral top edge 22.

As used herein, a PCD grade is a PCD material characterised in terms of the volume content and size of diamond grains, the volume content of interstitial regions between the diamond grains and composition of material that may be present within the interstitial regions. A grade of PCD material may be made by a process including providing an aggregate mass of diamond grains having a size distribution suitable for the grade, optionally introducing catalyst material or additive material into the aggregate mass, and subjecting the aggregated mass in the presence of a source of catalyst material for diamond to a pressure and temperature at which diamond is more thermodynamically stable than graphite and at which the catalyst material is molten. Under these conditions, molten catalyst material may infiltrate from the source into the aggregated mass and is likely to promote direct intergrowth between the diamond grains in a process of sintering, to form a PCD structure. The aggregate mass may comprise loose diamond grains or diamond grains held together by a binder material and said diamond grains may be natural or synthesised diamond grains. Different PCD grades may have different microstructures and different mechanical properties, such as elastic (or Young's) modulus E, modulus of elasticity, transverse rupture strength (TRS), toughness (such as so-called K₁C toughness), hardness, density and coefficient of thermal expansion (CTE). Different PCD grades may also perform differently in use. For example, the wear rate and fracture resistance of different PCD grades may be different.

All of the PCD grades may comprise interstitial regions filled with material comprising cobalt metal, which is an example of catalyst material for diamond.

The PCD structure 20 comprises one or more PCD grades.

As used herein, the term “stress state” refers to a compressive, unstressed or tensile stress state. Compressive and tensile stress states are understood to be opposite stress states from each other. In a cylindrical geometrical system, the stress states may be axial, radial or circumferential, or a net stress state.

With reference to FIG. 2, an example of a PCD structure 20 comprises at least two spaced-apart compressed regions 21 in compressive residual stress states and at least one tensioned region 22 in a tensile residual stress state. The tensioned region 22 is located between the compressed regions 21 and is joined to them.

Variations in mechanical properties of the PCD material such as density, elastic modulus, hardness and coefficient of thermal expansion (CTE) may be selected to achieve the configuration of a tensioned region between two compressed regions. Such variations may be achieved by means of variations in content of diamond grains, content and type of filler material, size distribution or mean size of the PCD grains, and using different PCD grades either on their own or in diamond mixes comprising a mixture of PCD grades.

With reference to FIG. 3, an example of a PCD element 10 comprises a PCD structure 20 integrally joined to a cemented carbide support body 30. The PCD structure 20 comprises several compressed regions 21 and several tensioned regions 22 in the form of alternating (or inter-leaved) strata or layers. The PCD element 10 may be substantially cylindrical in shape, with the PCD structure 20 located at a working end and defining a working surface 24. The PCD structure 20 may be joined to the support body 30 at a non-planar interface 25. The compressed and tensioned regions 21, 22 have a thickness in the range from about 5 microns to about 200 or, in some embodiments, 300 microns and may be arranged substantially parallel to the working surface 24 of the PCD structure 20. A substantially annular region 26 may be located around a non-planar feature 31 projecting from the support body 30.

With reference to FIG. 4, an example of a PCD element 10 comprises a PCD structure 20 integrally joined to a cemented carbide support body 30 at a non-planar interface 25 opposite a working surface 24 of the PCD structure 20. The PCD structure 20 may comprise about 10 to 20 alternating compressed and tensioned regions 21, 22 in the form of extended strata or layers. A region 26 that, in this embodiment, does not contain strata may be located adjacent the interface 25. The strata 21, 22 may be curved or bowed and yet generally aligned with the interface 25, and may intersect a side surface 27 of the PCD structure. Some of the strata may intersect the working surface 24.

In some embodiments, the region 26 may be of a substantially greater thickness than the individual strata or layers 21, 22 and, in some embodiments, the thickness of the region comprising the alternating layers 21, 22 may be of a greater thickness than the thickness of the region 26 adjacent the cemented carbide support body 30 which forms a substrate for the PCD material.

In some embodiments, the region 26 adjacent the support body 30 may include multiple layers or strata (not shown) that are of substantially greater thickness than the individual layers or strata 21, 22, for example, the layers 21, 22 may have a thickness in the range from about 5 to 200 microns, and the layers in the region 26 adjacent the support body 30 may have a thickness of greater than about 200 microns.

In some embodiments, such as those shown in FIGS. 1 to 4, the alternating strata, 21, 22 may have a thickness or thicknesses in the range of from about 5 to 300 microns with the diamond material being formed of PCD with two or more different average diamond grain sizes, for example a mixture of two or more grades of PCD. For example, strata 21 may be formed of an aggregated diamond mix having average diamond grain sizes A and B and strata 22 may also be formed of a diamond mix having average diamond grain sizes A and B but in a different ratio to that of strata 21. In an alternative embodiment, the strata 21 may be formed of a diamond mix having average diamond grain sizes A and B and the strata 22 may be formed of a diamond mix having an average diamond grain size C. It will be appreciated that any other sequence/mixture of two or more diamond grain sizes may be used to form the alternating layers 21, 22. In these embodiments, the region 26 adjacent the support body 30 may be formed of a single layer substantially thicker than the individual strata 21, 22, for example, greater than around 200 microns. Alternatively, the region 26 may be formed of multiple layers, individual layers or strata comprising diamond grains of average grain size A and B, and/or C as used to form the diamond mixes of the strata 21, 22 or another material or diamond grain size may be used to form the layers in this region 26 adjacent the support body 30.

In some embodiments, the diamond layers or strata 21, 22 and/or strata formed in region 26 adjacent the support body 30 (not shown), may include, for example, one or more of nanodiamond additions in the form of nanodiamond powder up to 20 wt %, salt systems, borides, metal carbides of Ti, V, Nb or any of the metals Pd or Ni.

In some embodiments, the strata 21, 22 and/or strata formed in region 26 adjacent the support body 30 may lie in a plane substantially perpendicular to the plane through which the longitudinal axis of the diamond construction 10 extends. The strata may be planar, curved, bowed, domed or distorted, for example, as a result of being subjected to ultra-high pressure during sintering. Alternatively, the alternating strata 21, 22 may be aligned at a predetermined angle to the plane through which the longitudinal axis of the diamond construction 10 extends to influence performance through crack propagation control.

With reference to FIG. 5, an example of a drill bit 60 for drilling into rock is shown as comprising example PCD elements 10 mounted onto a bit body 62. The PCD elements 10 are arranged so that the respective PCD structures 20 project from the bit body 62 for cutting the rock.

The grains of superhard material which are to form one or more layers or strata in any one or more of the regions, may be for example diamond grains or particles. In the starting mixture prior to sintering they may be, for example, bimodal, that is, the feed comprises a mixture of a coarse fraction of diamond grains and a fine fraction of diamond grains which are to form one or more of the alternating layers or strata. In some embodiments, the coarse fraction may have, for example, an average particle/grain size ranging from about 10 to 60 microns. By “average particle or grain size” it is meant that the individual particles/grains have a range of sizes with the mean particle/grain size representing the “average”. The average particle/grain size of the fine fraction is less than the size of the coarse fraction, for example between around 1/10 to 6/10 of the size of the coarse fraction, and may, in some embodiments, range for example between about 0.1 to 20 microns.

In some embodiments, the weight ratio of the coarse diamond fraction to the fine diamond fraction ranges from about 50% to about 97% coarse diamond and the weight ratio of the fine diamond fraction may be from about 3% to about 50%. In other embodiments, the weight ratio of the coarse fraction to the fine fraction will range from about 70:30 to about 90:10.

In further embodiments, the weight ratio of the coarse fraction to the fine fraction may range for example from about 60:40 to about 80:20.

In some embodiments, the particle size distributions of the coarse and fine fractions do not overlap and in some embodiments the different size components of the compact are separated by an order of magnitude between the separate size fractions making up the multimodal distribution.

The embodiments consists of at least a wide bi-modal size distribution between the coarse and fine fractions of superhard material, but some embodiments may include three or even four or more size modes which may, for example, be separated in size by an order of magnitude, for example, a blend of particle sizes whose average particle size is 20 microns, 2 microns, 200 nm and 20 nm.

Sizing of diamond particles/grains into fine fraction, coarse fraction, or other sizes in between, may be through known processes such as jet-milling of larger diamond grains and the like.

In embodiments where the superhard material is polycrystalline diamond material, the diamond grains used to form the polycrystalline diamond material may be natural or synthetic.

In some embodiments, the binder catalyst/solvent may comprise cobalt or some other iron group elements, such as iron or nickel, or an alloy thereof. Carbides, nitrides, borides, and oxides of the metals of Groups IV-VI in the periodic table are other examples of non-diamond material that might be added to the sinter mix. In some embodiments, the binder/catalyst/sintering aid may be Co.

The cemented metal carbide substrate may be conventional in composition and, thus, may be include any of the Group IVB, VB, or VIB metals, which are pressed and sintered in the presence of a binder of cobalt, nickel or iron, or alloys thereof. In some embodiments, the metal carbide is tungsten carbide.

In some embodiments, both the bodies of, for example, diamond and carbide material plus sintering aid/binder/catalyst are applied as powders and are sintered simultaneously in a single UHP/HT process. The alternating layers or strata of diamond, and mass of carbide to form the substrate are placed in an HP/HT reaction cell assembly and subjected to HP/HT processing. The HP/HT processing conditions selected are sufficient to effect intercrystalline bonding between adjacent grains of abrasive particles and, optionally, the joining of sintered particles to the cemented metal carbide support. In one embodiment, the processing conditions generally involve the imposition for about 3 to 120 minutes of a temperature of at least about 1200 degrees C. and an ultra-high pressure of greater than about 5 GPa.

In some embodiments, the substrate may be pre-sintered in a separate process before being bonded together in the HP/HT press during sintering of the ultrahard polycrystalline material.

In a further embodiment, both the substrate and a body of polycrystalline superhard material are pre-formed. For example, the bimodal or multimodal feed of ultrahard grains/particles with optional carbonate binder-catalyst also in powdered form are mixed together, and the mixture is packed in alternating layers or strata into an appropriately shaped canister and is then subjected to extremely high pressure and temperature in a press. Typically, the pressure is at least 5 GPa and the temperature is at least around 1200 degrees C. The preformed body of polycrystalline superhard material is then placed in the appropriate position on the upper surface of the preform carbide substrate (incorporating a binder catalyst), and the assembly is located in a suitably shaped canister. The assembly is then subjected to high temperature and pressure in a press, the order of temperature and pressure being again, at least around 1200 degrees C. and 5 GPa respectively. During this process the solvent/catalyst migrates from the substrate into the body of superhard material and acts as a binder-catalyst to effect intergrowth in the layer and also serves to bond the layer of polycrystalline superhard material to the substrate. The sintering process also serves to bond the body of superhard polycrystalline material to the substrate.

An further example method for making a PCD element is now described. Aggregate masses in the form of sheets containing diamond grains held together by a binder material may be provided. The sheets may be made by a method known in the art, such as by extrusion or tape casting methods, in which slurries comprising diamond grains having respective size distributions suitable for making the desired respective bimodal or multimodal PCD grades, and a binder material is spread onto a surface and allowed to dry. Other methods for making diamond-containing sheets may also be used, such as described in U.S. Pat. Nos. 5,766,394 and 6,446,740. Alternative methods for depositing diamond-bearing layers include spraying methods, such as thermal spraying. The binder material may comprise a water-based organic binder such as methyl cellulose or polyethylene glycol (PEG) and different sheets comprising diamond grains having different size distributions, diamond content or additives may be provided. For example, at least two sheets comprising diamond having different mean sizes may be provided and first and second sets of discs may be cut from the respective first and second sheets. The sheets may also contain catalyst material for diamond, such as cobalt, and/or additives for inhibiting abnormal growth of the diamond grains or enhancing the properties of the PCD material. For example, the sheets may contain about 0.5 weight percent to about 5 weight percent of vanadium carbide, chromium carbide or tungsten carbide. In one example, each of the sets may comprise about 10 to 20 discs.

A support body comprising cemented carbide in which the cement or binder material comprises a catalyst material for diamond, such as cobalt, may be provided. The support body may have a non-planar end or a substantially planar proximate end on which the PCD structure is to be formed and which forms the interface. A non-planar shape of the end may be configured to reduce undesirable residual stress between the PCD structure and the support body. A cup may be provided for use in assembling the diamond-containing sheets onto the support body. The first and second sets of discs may be stacked into the bottom of the cup in alternating order. In one version of the method, a layer of substantially loose diamond grains may be packed onto the uppermost of the discs. The support body may then be inserted into the cup with the proximate end going in first and pushed against the substantially loose diamond grains, causing them to move slightly and position themselves according to the shape of the non-planar end of the support body to form a pre-sinter assembly.

The pre-sinter assembly may be placed into a capsule for an ultra-high pressure press and subjected to an ultra-high pressure of at least about 5.5 GPa and a high temperature of at least about 1,300 degrees centigrade to sinter the diamond grains and form a PCD element comprising a PCD structure integrally joined to the support body. In one version of the method, when the pre-sinter assembly is treated at the ultra-high pressure and high temperature, the binder material within the support body melts and infiltrates the strata of diamond grains. The presence of the molten catalyst material from the support body is likely to promote the sintering of the diamond grains by intergrowth with each other to form an integral, stratified PCD structure.

In some versions of the method, the aggregate masses may comprise substantially loose diamond grains, or diamond grains held together by a binder material. The aggregate masses of multimodal grains may be in the form of granules, discs, wafers or sheets, and may contain catalyst material for diamond and/or additives for reducing abnormal diamond grain growth, for example, or the aggregated mass may be substantially free of catalyst material or additives. In some embodiments, the aggregate masses may be assembled onto a cemented carbide support body.

In some embodiments, the pre-sinter assembly may be subjected to a pressure of at least about 6 GPa, at least about 6.5 GPa, at least about 7 GPa or even at least about 7.5 GPa.

With reference to FIG. 6A, an example of a pre-sinter assembly 40 for making a PCD element may comprise a support body 30, a region 46 comprising diamond grains packed against a non-planar end of the support body 30, and a plurality of alternating diamond-containing aggregate masses in the general form of discs or wafers 41, 42 stacked on the region 46. In some versions, the aggregate masses may be in the form of loose diamond grains or granules. The pre-sinter assembly may be heated to remove the binder material comprised in the stacked discs.

With reference to FIG. 6B, an example of a PCD element 10 comprises a PCD structure 20 comprising a plurality of alternating strata 21, 22 formed of different respective multimodal grades of PCD material, and a portion 26 that does not comprise strata. The portion 26 may be cooperatively formed according to the shape of the non-planar end of the support body 30 to which it has integrally bonded during the treatment at the ultra-high pressure. The alternating strata 21, 22 of different grades of PCD or mixes of diamond grain sizes or grades are bonded together by direct diamond-to-diamond intergrowth to form an integral, solid and stratified PCD structure 20. The shapes of the PCD strata 21, 22 may be curved, bowed or distorted in some way as a result of being subjected to the ultra-high pressure. In some versions of the method, the aggregate masses may be arranged in the pre-sinter assembly to achieve various other configurations of strata within the PCD structure, taking into account possible distortion of the arrangement during the ultra-high pressure and high temperature treatment.

The strata 21, 22 may comprise different respective PCD grades as a result of the different mean diamond grain sizes of the strata. Different amounts of catalyst material may infiltrate into the different types of discs 41, 42 comprised in the pre-sinter assembly since they comprise diamond grains having different mean sizes, and consequently different sizes of spaces between the diamond grains. The corresponding alternating PCD strata 21, 22 may thus comprise different, alternating amounts of catalyst material for diamond. The content of the filler material in terms of volume percent within the tensioned region may be greater than that within each of the compressed regions.

In one example, the compressed strata may comprise diamond grains having mean size greater than the mean size of the diamond grains of the tensioned strata.

Whilst not wishing to be bound by a particular theory, when the stratified PCD structure is allowed to cool from the high temperature at which it was formed, the alternating strata containing different amounts of metal catalyst material may contract at different rates. This may be because metal contracts much more substantially than diamond does as it cools from a high temperature. This differential rate of contraction may cause adjacent strata to pull against each other, thus inducing opposing stresses in them.

The PCD element 10 described with reference to FIG. 6B may be processed by grinding to modify its shape to form a PCD element substantially as described with reference to FIG. 4. This may involve removing part of some of the curved strata to form a substantially planar working surface and a substantially cylindrical side surface. Catalyst material may be removed from a region of the PCD structure adjacent the working surface or the side surface or both the working surface and the side surface. This may be done by treating the PCD structure with acid to leach out catalyst material from between the diamond grains, or by other methods such as electrochemical methods. A thermally stable region, which may be substantially porous, extending a depth of at least about 50 microns or at least about 100 microns from a surface of the PCD structure, may thus be provided. Some embodiments with 50 to 80 micron thick layers in which this leach depth is around 250 microns have been shown to exhibit substantially improved performance, for example a doubling in performance after leaching over an unleached PCD product. In one example, the substantially porous region may comprise at most 2 weight percent of catalyst material.

The use of alternating layers or strata with different grain sizes through, for example, differences in binder content, may controllably give a different structure when acid leaching is applied to the PCD construction 10, especially for the embodiments in which the binder does not contain V and/or Ti. Such a structure may be created as a result of different residual tungsten in each layer during HCl acid leaching. In essence, the rate of leaching is likely to be different in each layer (unless HF-containing acid is used) and this may enable preferential leaching especially at the edges of the PCD material. This may be more pronounced for layers thicker than 120 microns. This is unlikely to occur if HF acid leaching were applied to the PCD material. The reason for this is that, in such a process, the HCl acid removes Co and leaves behind tungsten, whilst HF acid leaching would remove everything in the binder composition.

With reference to FIG. 7A, an example variant of a PCD structure 20 comprises at least three substantially planar strata 21, 22 strata arranged in an alternating configuration substantially parallel to a working surface 24 of the PCD structure 20 and intersecting a side surface 27 of the PCD structure.

With reference to FIG. 7B, an example variant of a PCD structure 20 comprises at least three strata 21, 22 strata arranged in an alternating configuration, the strata having a curved or bowed shape, with at least part of the strata inclined away from a working surface 24 and cutting edge 28 of the PCD structure.

With reference to FIG. 7C, an example variant of a PCD structure 20 comprises at least three strata 21, 22 strata arranged in an alternating configuration, at least part of the strata inclined away from a working surface 24 of the PCD structure and extending generally towards a cutting edge 28 of the PCD structure.

With reference to FIG. 7D, an example variant of a PCD structure 20 comprises at least three strata 21, 22 strata arranged in an alternating configuration, at least part of some of the strata being substantially aligned with a working surface 24 of the PCD structure and at least part of some of the strata generally aligned with a side surface 27 of the PCD structure. Strata may be generally annular of part annular and substantially concentric with a substantially cylindrical side surface 27 of the PCD structure 20.

The PCD structure may have a surface region proximate a working surface, the region comprising PCD material having a Young's modulus of at most about 1,050 MPa, or at most about 1,000 MPa. The surface region may comprise thermally stable PCD material.

Some examples of PCD structures may have at least 3, at least 5, at least 7, at least 10 or even at least 15 compressed regions, with tensioned regions located between them.

In some embodiments, each stratum or layer may have a thickness of at least about 5 microns, in others at least about 30 microns, in others at least about 100 microns, or in others at least about 200 microns. In some embodiments, each stratum or layer may have a thickness of at most about 300 microns or at most about 500 microns. In some example embodiments, each stratum or layer may have a thickness of at least about 0.05 percent, at least about 0.5 percent, at least about 1 percent or at least about 2 percent of a thickness of the PCD structure measured from a point on a working surface at one end to a point on an opposing surface. In some embodiments, each stratum or layer may have a thickness of at most about 5 percent of the thickness of the PCD structure.

As used herein, the term “residual stress state” refers to the stress state of a body or part of a body in the absence of an externally-applied loading force. The residual stress state of a PCD structure, including a layer structure may be measured by means of a strain gauge and progressively removing material layer by layer. In some examples of PCD elements, at least one compressed region may have a compressive residual stress of at least about 50 MPa, at least about 100 MPa, at least about 200 MPa, at least about 400 MPa or even at least about 600 MPa. The difference between the magnitude of the residual stress of adjacent strata may be at least about 50 MPa, at least about 100 MPa, at least about 200 MPa, at least about 400 MPa, at least about 600 MPa, at least about 800 MPa or even at least about 1,000 MPa. In one example, at least two successive compressed regions or tensioned regions may have different residual stresses. The PCD structure may comprise at least three compressed or tensioned regions each having a different residual compressive stress, the regions arranged in increasing or decreasing order of compressive or tensile stress magnitude, respectively.

In one example, each of the regions may have a mean toughness of at most 16 MPa·m½. In some embodiments, each of the regions may have a mean hardness of at least about 50 GPa, or at least about 60 GPa. Each of the regions may have a mean Young's modulus of at least about 900 MPa, at least about 950 MPa, at least about 1,000 or even at least about 1,050 MPa.

As used herein, “transverse rupture strength” (TRS) is measured by subjecting a specimen in the form of a bar having width W and thickness T to a load applied at three positions, two on one side of the specimen and one on the opposite side, and increasing the load at a loading rate until the specimen fractures at a load P. The TRS is then calculated based on the load P, dimensions of the specimen and the span L, which is the distance between the two load positions on one side. Such a measurement may also be referred to as a three-point bending test and is described by D. Munz and T. Fett in “Ceramics, mechanical properties, failure behaviour, materials selection” (1999, Springer, Berlin). The TRS corresponding to a particular grade of PCD material is measured measuring the TRS of a specimen of PCD consisting of that grade.

While the provision of a PCD structure with PCD strata having alternating compression and tensile stress states tends to increase the overall effective toughness of the PCD structure, this may have the effect of increasing the potential incidence of de-lamination, in which the strata may tend to come apart. While wishing not to be bound by a particular theory, de-lamination may tend to arise if the PCD strata are not sufficiently strong to sustain the residual stress between them. This effect may be ameliorated by selecting the PCD grades, and the PCD grade or grades of which the tensioned region in particular is formed, to have sufficiently high TRS. The TRS of the PCD grade or grades of which the tensioned region is formed should be greater than the residual tension that it may experience. One way of influencing the magnitude of the stress that a region may experience is by selecting the relative thicknesses of adjacent regions. For example, by selecting the thickness of a tensioned region to be greater than that of the adjacent compressive regions is likely to reduce the magnitude of tensile stress within the tensioned region.

The residual stress states of the regions may vary with temperature. In use, the temperature of the PCD structure may differ substantially between points proximate a cutting edge and points remote from the cutting edge. In some uses, the temperature proximate the cutting edge may reach several hundred degrees centigrade. If the temperature exceeds about 750 degrees centigrade, diamond material in the presence of catalyst material such as cobalt is likely to convert to graphite material, which is not desired. Therefore, in some uses, the alternating stress states in adjacent regions as described herein should be considered at a temperature of up to about 750 degrees centigrade.

The K1C toughness of a PCD disc is measured by means of a diametral compression test, which is described by Lammer (“Mechanical properties of polycrystalline diamonds”, Materials Science and Technology, volume 4, 1988, p. 23.) and Miess (Miess, D. and Rai, G., “Fracture toughness and thermal resistances of polycrystalline diamond compacts”, Materials Science and Engineering, 1996, volume A209, number 1 to 2, pp. 270-276).

Young's modulus is a type of elastic modulus and is a measure of the uni-axial strain in response to a uni-axial stress, within the range of stress for which the material behaves elastically. A preferred method of measuring the Young's modulus E is by means of measuring the transverse and longitudinal components of the speed of sound through the material, according to the equation E=2ρ·C_(T) ²(1+υ), where

υ=(1−2 (C _(T) /C _(L))²)/2-2(C _(T) /C _(L))²),

C_(L) and C_(T) are respectively the measured longitudinal and transverse speeds of sound through it and ρ is the density of the material. The longitudinal and transverse speeds of sound may be measured using ultrasonic waves, as is well known in the art. Where a material is a composite of different materials, the mean Young's modulus may be estimated by means of one of three formulas, namely the harmonic, geometric and rule of mixtures formulas as follows:

E=1/(f ₁ /E ₁ +f ₂ /E ₂)); E=E ₁ ^(f1) +E ₁ ^(f2.); and E=f ₁ E ₁ +f ₂ E ₂;

in which the different materials are divided into two portions with respective volume fractions of f₁ and f₂, which sum to one.

As used herein, the expression “formed of” means “consists of, apart from possible minor or non-substantial deviations in composition or microstructure”.

The hardness of cemented tungsten carbide substrate may be enhanced by subjecting the substrate to an ultra-high pressure and high temperature, particularly at a pressure and temperature at which diamond is thermodynamically stable. The magnitude of the enhancement of the hardness may depend on the pressure and temperature conditions. In particular, the hardness enhancement may increase the higher the pressure. Whilst not wishing to be bound by a particular theory, this is considered to be related to the Co drift from the substrate into the PCD during press sintering, as the extent of the hardness increase is directly dependent on the decrease of Co content in the substrate.

In embodiments where the cemented carbide substrate does not contain sufficient solvent/catalyst for diamond, and where the PCD structure is integrally formed onto the substrate during sintering at an ultra-high pressure, solvent/catalyst material may be included or introduced into the aggregated mass of diamond grains from a source of the material other than the cemented carbide substrate. The solvent/catalyst material may comprise cobalt that infiltrates from the substrate in to the aggregated mass of diamond grains just prior to and during the sintering step at an ultra-high pressure. However, in embodiments where the content of cobalt or other solvent/catalyst material in the substrate is low, particularly when it is less than about 11 weight percent of the cemented carbide material, then an alternative source may need to be provided in order to ensure good sintering of the aggregated mass to form PCD.

Solvent/catalyst for diamond may be introduced into the aggregated mass of diamond grains by various methods, including blending solvent/catalyst material in powder form with the diamond grains, depositing solvent/catalyst material onto surfaces of the diamond grains, or infiltrating solvent/catalyst material into the aggregated mass from a source of the material other than the substrate, either prior to the sintering step or as part of the sintering step. Methods of depositing solvent/catalyst for diamond, such as cobalt, onto surfaces of diamond grains are well known in the art, and include chemical vapour deposition (CVD), physical vapour deposition (PVD), sputter coating, electrochemical methods, electroless coating methods and atomic layer deposition (ALD). It will be appreciated that the advantages and disadvantages of each depend on the nature of the sintering aid material and coating structure to be deposited, and on characteristics of the grain.

In one embodiment, cobalt may be deposited onto surfaces of the diamond grains by first depositing a pre-cursor material and then converting the precursor material to a material that comprises elemental metallic cobalt. For example, in the first step cobalt carbonate may be deposited on the diamond grain surfaces using the following reaction:

Co(NO₃)₂+Na₂CO₃→CoCO₃+2NaNO₃

The deposition of the carbonate or other precursor for cobalt or other solvent/catalyst for diamond may be achieved by means of a method described in PCT patent publication number WO/2006/032982. The cobalt carbonate may then be converted into cobalt and water, for example, by means of pyrolysis reactions such as the following:

CoCO₃→CoO+CO₂

CoO+H₂→Co+H₂O

In another embodiment, cobalt powder or precursor to cobalt, such as cobalt carbonate, may be blended with the diamond grains. Where a precursor to a solvent/catalyst such as cobalt is used, it may be necessary to heat treat the material in order to effect a reaction to produce the solvent/catalyst material in elemental form before sintering the aggregated mass.

The practical use of cemented carbide grades with substantially lower cobalt content as substrates for PCD inserts is limited by the fact that some of the Co is required to migrate from the substrate into the PCD layer during the sintering process in order to catalyse the formation of the PCD. For this reason, it is more difficult to make PCD on substrate materials comprising lower Co contents, even though this may be desirable.

In some embodiments, the cemented carbide substrate may be formed of tungsten carbide particles bonded together by the binder material, the binder material comprising an alloy of Co, Ni and Cr. The tungsten carbide particles may form at least 70 weight percent and at most 95 weight percent of the substrate. The binder material may comprise between about 10 to 50 wt. % Ni, between about 0.1 to 10 wt. % Cr, and the remainder weight percent comprises Co. The size distribution of the tungsten carbide particles in the cemented carbide substrate ion some embodiments has the following characteristics:

-   -   fewer than 17 percent of the carbide particles have a grain size         of equal to or less than about 0.3 microns;     -   between about 20 to 28 percent of the tungsten carbide particles         have a grain size of between about 0.3 to 0.5 microns;     -   between about 42 to 56 percent of the tungsten carbide particles         have a grain size of between about 0.5 to 1 microns;     -   less than about 12 percent of the tungsten carbide particles are         greater than 1 micron; and     -   the mean grain size of the tungsten carbide particles is about         0.6±0.2 microns.

In some embodiments, the binder additionally comprises between about 2 to 20 wt. % tungsten and between about 0.1 to 2 wt. % carbon

A layer of the substrate adjacent to the interface with the body of polycrystalline diamond material may have a thickness of, for example, around 100 microns and may comprise tungsten carbide grains, and a binder phase. This layer may be characterised by the following elemental composition measured by means of Energy-Dispersive X-Ray Microanalysis (EDX):

-   -   between about 0.5 to 2.0 wt % cobalt;     -   between about 0.05 to 0.5 wt. % nickel;     -   between about 0.05 to 0.2 wt. % chromium; and     -   tungsten and carbon.

In a further embodiment, in the layer described above in which the elemental composition includes between about 0.5 to 2.0 wt % cobalt, between about 0.05 to 0.5 wt. % nickel and between about 0.05 to 0.2 wt. % chromium, the remainder is tungsten and carbon.

The layer of substrate may further comprise free carbon.

As used herein, the “mean free path” (MFP) of a composite material such as cemented carbide is a measure of the mean distance between the aggregate carbide grains cemented within the binder material. The mean free path characteristic of a cemented carbide material may be measured using a micrograph of a polished section of the material. For example, the micrograph may have a magnification of about 1000×. The MFP may be determined by measuring the distance between each intersection of a line and a grain boundary on a uniform grid. The matrix line segments, Lm, are summed and the grain line segments, Lg, are summed. The mean matrix segment length using both axes is the “mean free path”. Mixtures of multiple distributions of tungsten carbide particle sizes may result in a wide distribution of MFP values for the same matrix content. This is explained in more detail below.

In some embodiments, the substrate comprises Co, Ni and Cr.

The binder material for the substrate may include at least about 0.1 weight percent to at most about 5 weight percent one or more of V, Ta, Ti, Mo, Zr, Nb and Hf in solid solution.

To assist in improving thermal stability of the sintered structure, the catalysing material may be removed from a region of the polycrystalline layer adjacent an exposed surface thereof. Generally, that surface will be on a side of the polycrystalline layer opposite to the substrate and will provide a working surface for the polycrystalline diamond layer. Removal of the catalysing material may be carried out using methods known in the art such as electrolytic etching, and acid leaching and evaporation techniques. It has been found that multimodal distributions of some embodiments may assist in achieving a very high degree of diamond intergrowth while still maintaining sufficient open porosity to enable efficient leaching.

Embodiments are described in more detail below with reference to the following examples which are provided herein by way of illustration only and are not intended to be limiting.

EXAMPLE 1

A quantity of sub-micron cobalt powder sufficient to obtain 2 mass % in the final diamond mixture for forming one of the types of strata or layers was initially de-agglomerated in a methanol slurry in a ball mill with WC milling media for 1 hour. A fine fraction of diamond powder with an average grain size of 2 μm was then added to the slurry in an amount to obtain 10 mass % in the final mixture. Additional milling media was introduced and further methanol was added to obtain suitable slurry; and this was milled for a further hour. A coarse fraction of diamond, with an average grain size of approximately 20 μm was then added in an amount to obtain 88 mass % in the final mixture for forming one of the types of strata or layers. The slurry was again supplemented with further methanol and milling media, and then milled for a further 2 hours. The slurry was removed from the ball mill and dried to obtain the diamond powder mixture.

The diamond powder mixture was then placed in alternating layers or strata, alternating with another diamond mixture which may or may not be formed of a multimodal mixture of diamond grades into a suitable HpHT vessel, adjacent to a tungsten carbide substrate and sintered at a pressure of around 6.8 GPa and a temperature of about 1500° C.

EXAMPLE 2

A quantity of sub-micron cobalt powder sufficient to obtain 2.4 mass % in the final diamond mixture for forming one of the types of strata or layers was initially de-agglomerated in a methanol slurry in a ball mill with WC milling media for 1 hour. A fine fraction of diamond powder with an average grain size of 2 μm was then added to the slurry in an amount to obtain 29.3 mass % in the final mixture for forming one of the types of strata or layers. Additional milling media was introduced and further methanol was added to obtain a suitable slurry; and this was milled for a further hour. A coarse fraction of diamond, with an average grain size of approximately 20 μm was then added in an amount to obtain 68.3 mass % in the final mixture for forming one of the types of strata or layers. The slurry was again supplemented with further methanol and milling media, and then milled for a further 2 hours. The slurry was removed from the ball mill and dried to obtain the diamond powder mixture for forming one of the types of strata or layers.

The diamond powder mixture was then placed in alternating layers or strata, alternating with another diamond mixture which may or may not be formed of a multimodal mixture of diamond grades into a suitable HpHT vessel, adjacent to a tungsten carbide substrate and sintered at a pressure of around 6.8 GPa and a temperature of about 1500° C. In particular, first and second sheets, each containing diamond grains having a different mean size and held together by an organic binder were made by the tape casting method. This method involved providing respective slurries of diamond grains suspended in liquid binder, casting the slurries into sheet form and allowing them to dry to form self-supportable diamond-containing sheets. The diamond-containing wafers were placed into the cup, alternately stacked on top of each other with discs from the first and second sets inter-leaved. A layer of loose diamond grains to form a second region in which no alternating layers or strata were present having a mean size in the range from about 18 microns to about 25 microns was placed into the upturned cup, on top of the uppermost of the wafers, and the support body was inserted into the cup, with the non-planar end pushed against the layer.

The diamond content of the sintered diamond structure is greater than 90 vol % and the coarsest fraction of the distribution may, in some embodiments, be greater than 60 weight % or greater than weight 70%.

In polycrystalline diamond material, individual diamond particles/grains are, to a large extent, bonded to adjacent particles/grains through diamond bridges or necks. The individual diamond particles/grains retain their identity, or generally have different orientations. The average grain/particle size of these individual diamond grains/particles may be determined using image analysis techniques. Images are collected on a scanning electron microscope and are analysed using standard image analysis techniques. From these images, it is possible to extract a representative diamond particle/grain size distribution.

Generally, the body of polycrystalline diamond material will be produced and bonded to the cemented carbide substrate in a HPHT process. In so doing, it is advantageous for the binder phase and diamond particles in each layer or strata to be arranged such that the binder phase is distributed homogeneously and is of a fine scale.

The PCD element was processed by grinding and lapping to form a cutter element having a substantially planar working surface and cylindrical side. A cross-section through the PCD structure was then examined micro-structurally by means of a scanning electron microscope (SEM).

The homogeneity or uniformity of the sintered structure is defined by conducting a statistical evaluation of a large number of collected images. The distribution of the binder phase, which is easily distinguishable from that of the diamond phase using electron microscopy, can then be measured in a method similar to that disclosed in EP 0974566. This method allows a statistical evaluation of the average thicknesses of the binder phase along several arbitrarily drawn lines through the microstructure. This binder thickness measurement is also referred to as the “mean free path” by those skilled in the art. For two materials of similar overall composition or binder content and average diamond grain size, the material which has the smaller average thickness will tend to be more homogenous, as this implies a “finer scale” distribution of the binder in the layers or strata of the diamond phase. In addition, the smaller the standard deviation of this measurement, the more homogenous is the structure. A large standard deviation implies that the binder thickness varies widely over the microstructure, i.e. that the structure is not even, but contains widely dissimilar structure types.

The binder and diamond mean free path measurements were obtained for various samples in the manner set out below. Unless otherwise stated herein, dimensions of mean free path within the body of PCD material refer to the dimensions as measured on a surface of, or a section through, a body comprising PCD material and no stereographic correction has been applied. For example, the measurements are made by means of image analysis carried out on a polished surface, and a Saltykov correction has not been applied in the data stated herein.

In measuring the mean value of a quantity or other statistical parameter measured by means of image analysis, several images of different parts of a surface or section (hereinafter referred to as samples) are used to enhance the reliability and accuracy of the statistics. The number of images used to measure a given quantity or parameter may be, for example between 10 to 30. If the analysed sample is uniform, which is the case for PCD, depending on magnification, 10 to 20 images may be considered to represent that sample sufficiently well.

The resolution of the images needs to be sufficiently high for the inter-grain and inter-phase boundaries to be clearly made out and, for the measurements stated herein an image area of 1280 by 960 pixels was used. Images used for the image analysis were obtained by means of scanning electron micrographs (SEM) taken using a backscattered electron signal. The back-scatter mode was chosen so as to provide high contrast based on different atomic numbers and to reduce sensitivity to surface damage (as compared with the secondary electron imaging mode).

-   -   1. A sample piece of the PCD sintered body is cut using wire EDM         and polished. At least 10 back scatter electron images of the         surface of the sample are taken using a Scanning Electron         Microscope at 1000 times magnifications.     -   2. The original image was converted to a greyscale image. The         image contrast level was set by ensuring the diamond peak         intensity in the grey scale histogram image occurred between 10         and 20.     -   3. An auto threshold feature was used to binarise the image and         specifically to obtain clear resolution of the diamond and         binder phases.     -   4. The software, having the trade name analySIS Pro from Soft         Imaging System® GmbH (a trademark of Olympus Soft Imaging         Solutions GmbH) was used and excluded from the analysis any         particles which touched the boundaries of the image. This         required appropriate choice of the image magnification:     -   a. If too low then resolution of fine particles is reduced.     -   b. If too high then:     -   i. Efficiency of coarse grain separation is reduced.     -   ii. High numbers of coarse grains are cut by the boarders of the         image and hence less of these grains are analysed.     -   iii. Thus more images must be analysed to get a         statistically-meaningful result.     -   5. Each particle was finally represented by the number of         continuous pixels of which it is formed.     -   6. The AnalySIS software programme proceeded to detect and         analyse each particle in the image. This was automatically         repeated for several images.     -   7. Ten SEM images were analyzed using the grey-scale to identify         the binderpools as distinct from the other phases within the         sample. The threshold value for the SEM was then determined by         selecting a maximum value for binder pools content which only         identifies binder pools and excludes all other phases (whether         grey or white). Once this threshold value is identified it is         used to binarize the SEM image.)     -   8. One pixel thick lines were superimposed across the width of         the binarized image, with each line being five pixels apart (to         ensure the measurement is sufficiently representative in         statistical terms). Binder phase that are cut by image         boundaries were excluded in these measurements.     -   9. The distance between the binder pools along the superimposed         lines were measured and recorded—at least 10,000 measurements         were made per material being analysed. The median value for the         non-diamond phase mean free paths and the diamond phase mean         free paths were calculated. The term “median” in this context is         considered to have its conventional meaning, namely the         numerical value separating the higher half of the data sample         from the lower half.

Also recorded were the mean free path measurements at Q1 and Q3 for both the diamond and non-diamond phases.

Q1 is typically referred to as the first quartile (also called the lower quartile) and is the number below which lies the 25 percent of the bottom data. Q3 is typically referred to as the third quartile (also called the upper quartile) has 75 percent of the data below it and the top 25 percent of the data above it.

From this, it was determined that embodiments have:

alpha>=0.50 and beta<0.60,

where alpha is the non-diamond phase MFP median/(Q3−Q1), which gives a measure of “uniform binder pool size”; and beta=diamond MFP median/(Q3−Q1) which gives a measure of “wide grain size distribution”

The following clauses set out some of the possible combinations envisaged by the disclosure:

-   -   1. A PCD structure comprising a first layer or strata, a second         layer or strata and a third layer or strata; the second layer or         strata disposed between and bonded to the first and third layers         or strata by intergrowth of diamond grains; each layer or strata         being formed of a respective PCD grade or grades having a TRS of         at least 1,200 MPa or at least 1,600 MPa; the PCD grade or         grades comprised in the second layer or strata having a higher         coefficient of thermal expansion (CTE) than the respective PCD         grades of the first and third layers or strata. The second layer         or strata may comprise a PCD grade or grades having a CTE of at         least 4×10⁻⁶ mm/° C.     -   2. A PCD structure comprising a first and a third layer or         strata, each in a respective state of residual compressive         stress, and a second layer or strata in a state of residual         tensile stress and disposed between the first and third layer or         strata; the first, second and third layers or strata each formed         of one or more respective PCD grades and directly bonded to each         other by intergrowth of diamond grains; the PCD grades having         transverse rupture strength (TRS) of at least 1,200 MPa.     -   3. A PCD structure comprising a first layer or strata, a second         layer or strata and a third layer or strata; the second layer or         strata being disposed between and bonded to the first and third         layers or strata by intergrowth of diamond grains; each region         formed of one or more respective PCD grades comprising at least         85 volume percent diamond grains having a mean size of at least         0.1 micron and at most 30 micron; the PCD grade or grades         comprised in the second layer or strata containing a higher         content of metal than is contained in each of the respective PCD         grades comprised in the first and in the third layers or strata.         The PCD grade or grades comprised in the second layer or strata         may contain at least 9 volume percent metal.     -   4. A PCD structure comprising a first layer or strata, a second         layer or strata and a third layer or strata; the second layer or         strata being disposed between and bonded to the first and third         layers or strata by intergrowth of diamond grains; each layer or         strata being formed of one or more respective PCD grades having         a TRS of at least 1,200 MPa; the PCD grade or grades comprised         in the second layer or strata containing more metal than is         contained in each of the respective PCD grades comprised in the         first and in the third layers or strata. The PCD grade or grades         comprised in the second layer or strata may contain at least 9         volume percent metal.     -   5. In all of the combinations above numbered from 1 to 4, the         PCD structure may comprise a thermally stable region extending a         depth of at least 50 microns from a surface of the PCD         structure; in which the thermally stable region comprises at         most 2 weight percent of catalyst material for diamond.     -   6. In all of the combinations above numbered from 1 to 5, the         layers or strata may be in the form of strata arranged in an         alternating configuration to form an integral, stratified PCD         structure. The strata may have thickness of at least about 10         microns and at most about 500 microns, and the strata may be         generally planar, curved, bowed or domed.     -   7. In all of the combinations above numbered from 1 to 6, the         layers or strata may intersect a working surface or side surface         of the PCD structure. The PCD grade or grades comprised in the         first and third layers or strata may comprise diamond grains         having a different mean size than the diamond grains comprised         in the second layer or strata.     -   8. In all of the combinations above numbered from 1 to 7, the         volume or thickness of the second layer or strata may be greater         than the volume or thickness of the first layer or strata and         the volume or thickness of the third layer or strata.

A PCD element comprising a PCD structure bonded to a cemented carbide support body can be provided. The PCD element may be substantially cylindrical and have a substantially planar working surface, or a generally domed, pointed, rounded conical or frusto-conical working surface. The PCD element may be for a rotary shear (or drag) bit for boring into the earth, for a percussion drill bit or for a pick for mining or asphalt degradation.

While various embodiments have been described with reference to a number of examples, those skilled in the art will understand that various changes may be made and equivalents may be substituted for elements thereof and that these examples are not intended to limit the particular embodiments disclosed. For example, whilst the subsequent processing of the PCD element 10 such as leaching to remove catalyst material therefrom has been described with reference to the embodiment shown in FIG. 6B, such processing techniques could be applied to any of the embodiments. 

1. A superhard polycrystalline construction comprising a body of polycrystalline superhard material, the body of polycrystalline superhard material comprising a first region and a second region adjacent the first region, the second region being bonded to the first region by intergrowth of grains of superhard material; the first region comprising a plurality of alternating strata or layers, each stratum or layer having a thickness in the range of around 5 to 300 microns; the second region comprising a plurality of strata or layers, one or more strata or layers in the second region having a thickness greater than the thicknesses of the individual strata or layers in the first region, wherein: the alternating layers or strata in the first region comprise first layers or strata alternating with second layers or strata, the first layers or strata being in a state of residual compressive stress and the second layers or strata being in a state of residual tensile stress; one or more of the layers or strata in the first or second regions comprises: a mass of superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, the superhard grains having an associated mean free path; and a non-superhard phase at least partially filling a plurality of the interstitial regions and having an associated mean free path; the median of the mean free path associated with the non-superhard phase divided by (Q3−Q1) for the non-superhard phase being greater than or equal to 0.50, where Q1 is the first quartile and Q3 is the third quartile; and the median of the mean free path associated with the superhard grains divided by (Q3−Q1) for the superhard grains being less than 0.60.
 2. A superhard polycrystalline construction according to claim 1, wherein the superhard grains comprise natural and/or synthetic diamond grains, the superhard polycrystalline construction forming a polycrystalline diamond construction.
 3. A superhard polycrystalline construction according to claim 1, wherein the non-superhard phase comprises a binder phase.
 4. A superhard polycrystalline construction according to claim 3, wherein the binder phase comprises cobalt, and/or one or more other iron group elements, such as iron or nickel, or an alloy thereof, and/or one or more carbides, nitrides, borides, and oxides of the metals of Groups IV-VI in the periodic table.
 5. A superhard polycrystalline construction according to claim 1, further comprising a cemented carbide substrate bonded to the body of polycrystalline material along an interface.
 6. A superhard polycrystalline construction according to claim 5, wherein the cemented carbide substrate comprises tungsten carbide particles bonded together by a binder material, the binder material comprising an alloy of Co, Ni and Cr.
 7. A superhard polycrystalline construction according to claim 6, wherein the tungsten carbide particles form at least 70 weight percent and at most 95 weight percent of the substrate; the binder material comprising between about 10 to 50 wt. % Ni, between about 0.1 to 10 wt. % Cr, and the remainder weight percent comprising Co; and wherein the size distribution of the tungsten carbide particles in the cemented carbide substrate has the following characteristics: fewer than 17 percent of the tungsten carbide particles have a grain size of equal to or less than about 0.3 microns; between about 20 to 28 percent of the tungsten carbide particles have a grain size of between about 0.3 to 0.5 microns; between about 42 to 56 percent of the tungsten carbide particles have a grain size of between about 0.5 to 1 microns; less than about 12 percent of the tungsten carbide particles are greater than 1 micron; and the mean grain size of the tungsten carbide particles is about 0.6±0.2 microns.
 8. A superhard polycrystalline construction according to claim 7, wherein the binder additionally comprises between about 2 to 20 wt. % tungsten and between about 0.1 to 2 wt. % carbon.
 9. A superhard polycrystalline construction according to claim 1, wherein each stratum or layer in the first region has a thickness in the range of around 30 to 300 microns, or around 30 to 200 microns.
 10. A superhard polycrystalline construction according to claim 1, wherein the strata or layers in the second region have a thickness of greater than around 200 microns.
 11. A superhard polycrystalline construction according to claim 1, wherein the layers or strata in the first region comprise two or more different average diamond grain sizes.
 12. A superhard polycrystalline construction comprising a first region and a second region adjacent the first region, the second region being bonded to the first region by intergrowth of diamond grains; the first region comprising a plurality of alternating strata or layers, each layer or stratum in the first region having a thickness in the range of around 5 to 300 microns; one or more of the layers or strata in the first region and/or the second region comprises: a mass of superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, the superhard grains having an associated mean free path; and a non-superhard phase at least partially filling a plurality of the interstitial regions and having an associated mean free path; the median of the mean free path associated with the non-superhard phase divided by (Q3−Q1) for the non-superhard phase being greater than or equal to 0.50, where Q1 is the first quartile and Q3 is the third quartile; and the median of the mean free path associated with the superhard grains divided by (Q3−Q1) for the superhard grains being less than 0.60.
 13. A superhard polycrystalline construction according to claim 1, wherein the first region comprises an external working surface forming the initial working surface of the superhard polycrystalline construction in use.
 14. A superhard polycrystalline construction according to claim 12, wherein the second region has a thickness greater than the thickness of the individual strata or layers in the first region.
 15. A superhard polycrystalline construction according to claim 12, wherein the second region comprises a plurality of layers or strata.
 16. A superhard polycrystalline construction according to claim 12, wherein the alternating layers or strata comprise first layers or strata alternating with second layers or strata, the first layers or strata being in a state of residual compressive stress and the second layers or strata being in a state of residual tensile stress.
 17. A superhard polycrystalline construction according to claim 1, wherein layers or strata in the first region and/or the second region comprise one or more of: up to 20 wt % nanodiamond additions in the form of nanodiamond powder grains; salt systems; borides or metal carbides of at least one of Ti, V, or Nb; or at least one of the metals Pd or Ni.
 18. A superhard polycrystalline construction according to claim 1, wherein the superhard polycrystalline construction has a longitudinal axis, the layers or strata in the first region and/or the second region lying in a plane substantially perpendicular to the plane through which the longitudinal axis of the superhard polycrystalline construction extends.
 19. A superhard polycrystalline construction according to claim 1, wherein the layers or strata are substantially planar, curved, bowed or domed.
 20. A superhard polycrystalline construction according to claim 1, wherein the superhard polycrystalline construction has a longitudinal axis, the layers or strata in the first region and/or the second region lying in a plane at an angle to the plane through which the longitudinal axis of the PCD structure extends.
 21. A superhard polycrystalline construction according to claim 1, wherein the volume of the first region is greater than the volume of the second region.
 22. A superhard polycrystalline construction according to claim 1, wherein one or more of the strata or layers intersect a working surface or side surface of the superhard polycrystalline construction.
 23. A superhard polycrystalline construction according to claim 1, wherein each strata or layer is formed of one or more respective PCD grades having a TRS of at least 1,000 MPa; the PCD grade or grades in adjacent strata or layers having a different coefficient of thermal expansion (CTE).
 24. A superhard polycrystalline construction according to claim 23, wherein one or more of the strata or layers comprise a PCD grade or grades having a CTE of at least 3×10−6 mm/oC.
 25. A superhard polycrystalline construction as claimed in claim 1, wherein at least a portion of the first region is substantially free of a catalyst material for diamond, said portion forming a thermally stable region.
 26. A superhard polycrystalline construction as claimed in claim 25, wherein the thermally stable region extends a depth of at least 50 microns from a surface of the superhard polycrystalline construction.
 27. A superhard polycrystalline construction as claimed in claim 25, wherein the thermally stable region comprising at most 2 weight percent of catalyst material for diamond.
 28. A superhard polycrystalline construction for a rotary shear bit for boring into the earth, or for a percussion drill bit, comprising a superhard polycrystalline construction as claimed in claim 1 bonded to a cemented carbide support body.
 29. A method of forming a superhard polycrystalline construction, comprising: providing a mass of grains of superhard material and arranging the mass of superhard grains to form a first region comprising a plurality of alternating strata or layers, each stratum or layer having a respective first fraction having a first average size and a second fraction having a second average size, and providing a further mass of grains of superhard material to form a second region adjacent the first region to form a pre-sinter assembly; and treating the pre-sinter assembly in the presence of a catalyst/solvent material for the superhard grains at an ultra-high pressure of around 5.5 GPa or greater and a temperature at which the superhard material is more thermodynamically stable than graphite to sinter together the grains of superhard material to form a polycrystalline superhard construction, the superhard grains exhibiting inter-granular bonding and defining a plurality of interstitial regions therebetween, a non-superhard phase at least partially filling a plurality of the interstitial regions; the second region being bonded to the first region by intergrowth of grains of superhard material; the first region a thickness in the range of around 5 to 300 microns; wherein: the alternating layers or strata in the first region comprise first layers or strata alternating with second layers or strata, the first layers or strata being in a state of residual compressive stress and the second layers or strata being in a state of residual tensile stress; the median of the mean free path associated with the non-superhard phase divided by (Q3−Q1) for the non-superhard phase is greater than or equal to 0.50, where Q1 is the first quartile and Q3 is the third quartile of the mean free path measurements associated with the non-superhard phase; and the median of the mean free path associated with the superhard grains divided by (Q3−Q1) for the superhard grains is less than 0.60, where Q1 is the first quartile and Q3 is the third quartile of the mean free path measurements associated with the superhard grains.
 30. The method of claim 29, wherein, the step of providing a mass of grains of superhard material comprises providing a mass of diamond grains having a first fraction having a first average size and a second fraction having a second average size, the first fraction having an average grain size ranging from about 10 to 60 microns, and the second fraction having an average grain size less than the size of the first fraction.
 31. The method of claim 30, wherein the second fraction has an average grain size between around 1/10 to 6/10 of the size of the first fraction.
 32. The method of claim 29, wherein the average grain size of the first fraction is between around 10 to 60 microns, and the average grain size of the second fraction is between about 0.1 to 20 microns.
 33. The method of claim 29, wherein the weight ratio of the first fraction to the second fraction ranges from about 50% to about 97%, the weight % of the second fraction ranging from about 3% to about 50 weight %.
 34. The method of claim 33, wherein the ratio by weight percent of the first fraction to the second fraction is around 60:40.
 35. The method of claim 33, wherein the ratio by weight percent of the first fraction to the second fraction is around 70:30.
 36. The method of claim 33, wherein the ratio by weight percent of the first fraction to the second fraction is around 90:10.
 37. The method of claim 33, wherein the ratio by weight percent of the first fraction to the second fraction is around 80:20.
 38. The method of claim 29, wherein the step of providing a mass of grains of superhard material comprises providing a mass of grains in which the grain size distributions of the first and second fractions do not overlap.
 39. The method of claim 29, wherein the step of providing a mass of grains of superhard material comprises providing three or more grain size modes to form a multimodal mass of grains comprising a blend of grain sizes having associated average grain sizes.
 40. The method of claim 29, wherein the average grain sizes of the fractions is separated by an order of magnitude.
 41. The method of claim 39, wherein the mass of superhard grains comprises a first fraction having an average grain size of around 20 microns, a second fraction having an average grain size of around 2 microns, a third fraction having an average grain size of around 200 nm and a fourth fraction having an average grain size of around 20 nm.
 42. A tool comprising a superhard polycrystalline construction according to claim 1, the tool being for cutting, milling, grinding, drilling, earth boring, rock drilling or other abrasive applications.
 43. A tool according to claim 42, wherein the tool comprises a drill bit for earth boring or rock drilling.
 44. A tool according to claim 42, wherein the tool comprises a rotary fixed-cutter bit for use in oil and gas drilling.
 45. A tool according to claim 42, wherein the tool is a rolling cone drill bit, a hole opening tool, an expandable tool, a reamer or other earth boring tools.
 46. A drill bit or a cutter or a component therefor comprising the superhard polycrystalline construction according to claim
 1. 47-48. (canceled) 